A conventional projection-exposure apparatus employing an electron beam is schematically depicted in FIG. 3. Such an apparatus is used to transfer a pattern, defined on a reticle or mask (hereinafter simply referred to as a "mask") 1 onto a photosensitive substrate 5 (e.g., semiconductor wafer). The various components of the FIG. 3 apparatus are arranged along an optical axis AX.
The electron beam EB is emitted from an electron gun (not shown, but understood to be located just upstream of a crossover CO1 at which an image of the electron gun is formed). The electron beam EB is converged by an illumination lens 50 and deflected by deflectors 51 onto a subfield 1a of the mask 1. Because the mask 1 comprises multiple individual subfields 1a each defining a respective portion of the pattern, the mask is termed a "segmented" mask.
The electron beam EB illuminating the subfield 1a typically has a square or rectangular transverse profile, and includes an on-axis component EB1, and off-axis components represented by components EB2, EB3. The on-axis component EB1 strikes the center of the subfield 1a, while the representative off-axis components EB2, EB3 are directed to the left and right portions, respectively, of the subfield 1a in the figure. The bold line in the middle of the on-axis component EB1 indicates the general trajectory of the electron beam irradiating the subfield 1a.
The electron beam EB, after having passed through the subfield 1a of the mask 1, is focused on the photosensitive substrate 5 by first and second projection lenses 3, 4, respectively. The projection lenses 3, 4 form an image of the subfield 1a on a corresponding area of the photosensitive substrate 5.
The apparatus shown in FIG. 3 is termed a "VAL" (Variable Axis Lens) system that includes VAL deflectors 31, 32, 33, 34 and VAL focus-correction lenses 35, 36, 37, 38. Energizing any of the VAL deflectors 31-34 generates a corresponding magnetic field having a magnitude proportional to a(dB(Z)/dZ); similarly, energizing any of the VAL focus-correction lenses 35-38 generates a corresponding magnetic field having a magnitude proportional to a.sup.2 (d.sup.2 B/dZ.sup.2), wherein B(Z) is the magnetic flux distribution of the on-axis magnetic field generated by the respective projection lenses 3, 4, and "a" is the lateral deflection of the electron beam EB imparted by the deflectors 51 (i.e., the lateral distance from the optical axis AX to the center of the sub-field 1a). Thus, the magnetic field generated by the projection lenses 3, 4 (the field normally extending along the optical axis AX) is laterally shifted to the deflected trajectory axis (i.e., the trajectory of the electron beam EB laterally offset from the optical axis AX). Because the deflected trajectory axis equivalently traces along the common axis of the projection lenses 3, 4, the aberration exhibited by the projection lenses 3, 4 with respect to a laterally shifted beam is substantially the same as when the electron beam is propagating on-axis. Such an arrangement reportedly reduces lens aberrations compared to a lens arrangement lacking VAL deflectors 31-34 and VAL focus-correction lenses 35-38.
Further with respect to the FIG. 3 apparatus, a deflector 41 deflects the electron beam EB toward the optical axis AX to direct the beam EB to an aperture 6 positioned at a second crossover CO2 located between the mask 1 and the substrate 5. A deflector 42 further deflects the electron beam EB, that has passed through the aperture 6, such that the trajectory of the electron beam EB becomes parallel to the optical axis. The electron beam EB then strikes the substrate 5 at a normal angle relative to the surface of the substrate.
The mask 1 is movably supported by a mask stage MS and the substrate 5 is movably supported by a wafer stage WS. During exposure of the substrate 5 with the mask pattern, the mask stage MS and wafer stage WS are continuously moved in opposite directions relative to each other (e.g., opposite directions perpendicular to the plane of the page). Meanwhile, the electron beam EB is deflected in a direction perpendicular to the movement directions of the stages MS, WS (e.g., in horizontal directions parallel to the plane of the page) for each subfield 1a. While moving the mask stage MS and wafer stage WS in such a manner, the pattern portions in the subfields of the mask 1 are successively transferred onto the corresponding areas of the substrate 5, ultimately forming the entire mask pattern on the substrate 5.
The greater the width of each subfield (i.e., the left-to-right dimension of each subfield 1a shown in FIG. 3), the fewer subfields required to fully define the mask pattern, the fewer subfields that need to be illuminated by the electron beam EB, and the fewer back-and-forth motions of the stages MS, WS required to reproduce the entire mask pattern on the substrate. Hence, larger subfields can result in a shorter amount of time needed for each exposure of a complete mask pattern on the substrate 5 (termed greater "throughput" in the art).
However, according to the scheme shown in FIG. 3, certain subfields la will be located remotely from the optical axis AX. If such a distant subfield is irradiated and projected onto the substrate 5, deflection aberrations caused by the deflectors 41, 42 can be unacceptably large despite whatever contributions the VAL system makes to reducing aberrations in the projection lenses 3, 4. The increased aberration is proportional to the magnitude of lateral deflection imparted by the deflectors 41, 42.
Therefore, the greater the lateral deflection of the electron beam EB imparted by the deflector 51, the greater the image blur due to deflection aberrations. Consequently, the desired resolution for pattern transfer cannot be achieved using such a conventional apparatus.